This invention relates to an optical sensor arrangement for tracking a star and, more particularly, to a device for compensating for dark currents of optical sensor elements used in the optical sensor arrangement as an optical sensor which is typically a charge coupled device (CCD) comprising such optical sensor elements in a two-dimensional array. The star is preferably a fixed star.
Such an optical sensor arrangement is used in a spacecraft which may be an artificial satellite. For use typically in a spacecraft, an image pickup system is disclosed in U.S. Pat. No. 5,027,199 which was issued to the present inventor and assigned to the instant assignee. The image pickup system is for dealing with a terrestrial objective zone and is appreciably different in this respect from the optical sensor arrangement under consideration. This patent will, however, be herein incorporated by reference.
An excellent star tracker is revealed in Japanese Patent Prepublication (A) No. 226,398 of 1985 for a patent application filed in Japan in relation to a prior invention of YOKOTA-Toyohati (transliteration according to ISO 3602) by NEC Corporation, the present assignee. In the manner which will later be described in greater detail, the star tracker comprises a shutter in an optical path from an objective star to a charge coupled device of the star tracker. A shutter driver is controlled by a driver control circuit to drive the shutter in first through third modes of operation. As described heretobefore, the charge coupled device is used as an optical sensor and comprises two-dimensionally arranged optical sensor elements which may alternatively be called pixels.
In the first mode, the shutter is put away of the optical path. An optical system of the star tracker focusses the starlight as a real image of the objective star on a certain number of pixels, such as 1,000 by 1,000 pixels, of the charge coupled device. Responsive to the real image, the optical sensor produces a first sensor output signal. In the second mode, the optical path is interrupted by the shutter. The optical sensor produces a second sensor output signal which is equal to a sum of dark currents of the pixels. Responsive to the first and the second sensor output signals, a processing unit of the star tracker decides a dark current component of the above-mentioned small number of pixels. In the third mode, the shutter is again put aside. The optical sensor produces a third sensor output signal. Using the dark current component and the third sensor output signal, the processing unit exactly determines a true brightness of the objective star.
In this manner, the star tracker can compensate for dark currents of the optical sensor elements and for fluctuation in the dark currents among the optical sensor elements. Consequently, the star tracker is stably operable and can precisely determine an exact position of the objective star on the celestial sphere.
The star tracker must, however, comprise the shutter, the shutter driver, and the driver control circuit. As a consequence, the star tracker is bulky in size and heavy in weight. This is objectionable for use in a spacecraft. Moreover, the shutter is liable to misoperation. This gives an objectionable reliability to the star tracker.